This invention relates to an etching process.
Existing methods of etching metal surfaces to prepare them for the adhesion thereto of some form of coating include the physical roughening of the surface by grit blasting, wheel sanding, wire brushing and directional grinding. These existing methods have various disadvantages associated with them. Gritblasting is a slow and expensive process, while the other processes have to be carefully controlled as severe roughening of the surfaces must be avoided.
The use of ferric chloride, applied as a solution to a metal surface, to roughen the surface has also been proposed. It has however been discovered that the action of ferric chloride is very dependant upon the type of degreasing that is carried out and also that certain metals, including die-cast aluminium, are not etched by the application of ferric chloride to them. The process also does not provide the flexibility required to vary the etch depth and the etch profile.
A further problem hitherto associated with the use of ferric chloride as an etching agent, is that the ferric ions in solution, an integral component of the etching process, become reduced during the etching process to ferrous ions and then finally to iron with a simultaneous reduction in the etching power of the solution. The precipitation of iron from the etching solution indicates the total depletion of ferric chloride in the solution and the end of its use as an etching agent. At this stage, the solution must be discarded as waste. However, the pollutants in this solution make its disposal an expensive and hazardous exercise.